Crontabs: Difference between revisions

* a '''number''' indicates the exact time at which an action should occur. For example, if the number '''5''' was placed in the minute section, the specified action would be taken at the fifth minute of every hour.

* an '''asterisk''' ('''*''') stands for every possible value of the relevant time unit. For example, if placed in the hour section, it would indicate that an action should be taken every hour.

* two numbers separated by a '''dash'''('''-''') indicate a range of numbers. For example, if '''4-7''' was placed in the month section, it would indicate than the specified action should take place every fourth, fifth, sixth and seventh month of a year.

* numbers separated by '''commas''' (''',''') indicate multiple specific time stamps at which an action should occur. For example, if '''6,9,10''' was placed in the hour section, the specified action would be taken at 6:00 AM, 9:00 AM and 10 AM every day. Ranges of numbers can also be separated by commas. For instance, '''5-10,20-25''' placed in the minute section would indicate that an action must occur every fifth through tenth minute as well as every twentieth through twenty fifth minute.

To edit crontab, use '''crontab -e'''. This is analogous to using the '''vi''' command with the ''/etc/crontabs/root'' file, so the same rules  apply here as when editing with ''vi''. Type ''crontab -e'', press "Enter" and a text editor for the crontab file will open. To start editing, press the "I" key on your keyboard; you can then edit the crontab file much like with a regular text editor (except you can't use your mouse pointer to place the text cursor anywhere you want; use the arrow keys on your keyboard instead). To save changes after editing, press the "Escape" ("Esc") button on your keyboard, type ''':x''' and press "Enter". To cancel changes and exit the editor, press "'''Control'''" ('''Ctrl''') '''+''' "'''C'''" on your keyboard.

A simpler yet more restricted method of editing would be using the '''echo''' command to add lines to the crontab file. It can be used in such a manner: '''echo "Hello, world" >> /etc/crontabs/root'''. This command will add the text '''Hello, world''' to the crontab file, i.e., it adds whatever is specified within the quotation marks (" ") after the ''echo'' command to the file that is specified after the double '''more than''' ('''>>''') symbol. For example, to add a rule that reboots the router everyday at midnight, we would have to use:

Using the command in such a way simply adds a new line with the denoted text at the end of the specified file. This is convenient when you just want to add a new rule(s) quickly, but unlike ''vi'' or ''crontab -e'' it doesn't offer any other editing capabilities. So if you want edit or remove existing rules, this method will not offer a solution and ''crontab -e'' should be used instead.

This section will provide some crontab usage examples in the hopes of helping you getting the hang of the system or even finding one that you could use. The examples provided here will be examined thoroughly but the editing method will not be discussed; you can read up on that in the section above.

* 45 - the action must take place at minute 45 of the hour

To sum up, the first five segments denote the frequency of the sim_switch command. The option '''change''' specifies that the SIM card that is currently in use will be switched. So the entire entry will perform a SIM switch to the opposite SIM card every weekday at 6:45 PM.

For this example we'll configure a system that launches an OpenVPN server at the start of a workday (8 AM) and shuts down said server at the end of a workday (6 PM). For this we'll need to add two rules. Each rule will require multiple commands to be executed. Crontab can launch multiple commands the same way as a single command, only the '''&&''' separator should be used to separate the different commands:  

* 0/0 - the action must take place at minute 0 (just as the hour changes)

* 8/18 - the action must take place at 8 AM and 6 PM respectively

* 1-5 - the action must take place every Monday - Friday

* openvpn.7365727665725F64656D6F.enable='''1/0''' - enables/disables OpenVPN

* /etc/init.d/openvpn restart - restarts OpenVPN service

* && - used to separate different commands

The first line enables the OpenVPN server at 8 AM, the second disables it at 6 PM. In this example we used UCI to change the state of the OpenVPN server. You find more information on UCI '''[[UCI command usage|here]]'''.